Editing Regenerative fuel cell (section)

==Solid oxide regenerative fuel cell==
One example of RFC is solid oxide regenerative fuel cell. [[Solid oxide fuel cell]] operates at high temperatures with high fuel-to-electricity conversion ratios and it is a good candidate for high temperature electrolysis.<ref name="Laguna-Bercero2010">{{cite journal|last1=Laguna-Bercero|title=Performance and Aging ofMicrotubular YSZ-based Solid Oxide Regenerative Fuel Cells|journal=Fuel Cells|date=30 July 2010|doi=10.1002/fuce.201000069|first1=M. A.|last2=Campana|first2=R.|last3=Larrea|first3=A.|last4=Kilner|first4=J. A.|last5=Orera|first5=V. M.|volume=11|pages=116–123|hdl=10261/53668|s2cid=33333495 |url=https://digital.csic.es/bitstream/10261/53668/4/Performance%20and%20Aging%20of%20Microtubular.pdf|hdl-access=free}}</ref>  Less electricity is required for electrolysis process in solid oxide regenerative fuel cells (SORFC) due to high temperature.

The electrolyte can be O<sup>2−</sup> conducting and/or proton (H<sup>+</sup>) conducting. The state of the art for O<sup>2−</sup> conducting yttria stabilized zirconia (YSZ) based SORFC using Ni–YSZ as the hydrogen electrode and LSM (or LSM–YSZ) as the oxygen electrode has been actively studied.<ref name="Laguna-Bercero2010"/> Dönitz and Erdle reported on the operation of YSZ electrolyte cells with current densities of 0.3 A cm<sup>−2</sup> and 100% Faraday efficiency at only 1.07 V.<ref>{{cite journal |last1=Dönitz |first1=W. |last2=Erdle |first2=E. |title=High-temperature electrolysis of water vapor—status of development and perspectives for application |journal=International Journal of Hydrogen Energy |year=1985 |volume=10 |issue=5 |pages=291–295 |doi=10.1016/0360-3199(85)90181-8 |bibcode=1985IJHE...10..291D }}</ref> The recent study by researchers from Sweden shows that ceria-based composite electrolytes, where both proton and oxide ion conductions exist, produce high current output for fuel cell operation and high hydrogen output for electrolysis operation.<ref>{{cite journal|last=zhu|first=Bin |author2=Ingvar Albinsson |author3=Camilla Andersson |author4=Karin Borsand |author5=Monika Nilsson |author6=Bengt-Erik Mellander|title=Electrolysis studies based on ceria-based composites|journal=Electrochemistry Communications|date=20 February 2006|volume=8|issue=3|pages=495–498|doi=10.1016/j.elecom.2006.01.011}}</ref> Zirconia doped with scandia and ceria (10Sc1CeSZ) is also investigated as potential electrolyte in SORFC for [[hydrogen production]] at intermediate temperatures (500-750&nbsp;°C). It is reported that 10Sc1CeSZ shows good behavior and produces high current densities, with suitable electrodes.<ref>{{cite journal|last=Laguna-Bercero|first=M.A |author2=S.J. Skinnera |author3=J.A. Kilner|title=Performance of solid oxide electrolysis cells based on scandia stabilised zirconia|journal=Journal of Power Sources|date=1 July 2009|volume=192|issue=1|pages=126–131|doi=10.1016/j.jpowsour.2008.12.139|bibcode=2009JPS...192..126L|hdl=10044/1/13889 |url=http://spiral.imperial.ac.uk/bitstream/10044/1/13889/2/Journal%20of%20Power%20Sources_192_1_2009.pdf|hdl-access=free}}</ref>

Current density–voltage ({{math|j-V}}) curves and impedance spectra are investigated and recorded. Impedance
spectra are realized applying an ac current of 1–2A RMS (root-mean-square) in the frequency range from 30&nbsp;kHz
to 10<sup>−1</sup> Hz. Impedance spectra shows that the resistance is high at low frequencies (<10&nbsp;kHz) and near zero at high frequencies (>10&nbsp;kHz).<ref>{{cite journal|last=Brisse|first=Annabelle |author2=Josef Schefold |author3=Mohsine Zahida|title=High temperature water electrolysis in solid oxide cells|journal=International Journal of Hydrogen Energy|date=October 2008|volume=33|issue=20|pages=5375–5382|doi=10.1016/j.ijhydene.2008.07.120|bibcode=2008IJHE...33.5375B }}</ref>  Since high frequency corresponds to electrolyte activities, while low frequencies corresponds to electrodes process, it can be deduced that only a small fraction of the overall resistance is from the electrolyte and most resistance comes from anode and cathode. Hence, developing high performance electrodes are essential for high efficiency SORFC. Area specific resistance can be obtained from the slope of {{math|j-V}} curve. Commonly used/tested electrodes materials are nickel/zirconia cermet (Ni/YSZ) and lanthanum-substituted strontium titanate/ceria composite for SORFC cathode, and lanthanum strontium manganite (LSM) for SORFC anode. Other anode materials can be lanthanum strontium ferrite
(LSF), lanthanum strontium copper ferrite and lanthanum strontium cobalt ferrite. Studies show that Ni/YSZ electrode was less active in reverse fuel cell operation than in fuel cell operation, and this can be attributed to a diffusion-limited process in the electrolysis direction, or its susceptibility to aging in a high-steam environment,
primarily due to coarsening of nickel particles.<ref>{{cite journal|last1=Marina|first1=O. A.|last2=Pederson|first2=L. R.|last3=Williams|first3=M. C.|last4=Coffey|first4=G. W.|last5=Meinhardt|first5=K. D.|last6=Nguyen|first6=C. D.|last7=Thomsen|first7=E. C.|title=Electrode Performance in Reversible Solid Oxide Fuel Cells|journal=Journal of the Electrochemical Society|date=22 March 2007|doi=10.1149/1.2710209|url=http://availabletechnologies.pnnl.gov/media/103_311200832146.pdf|volume=154|issue=5|pages=B452|bibcode=2007JElS..154B.452M }}</ref> Therefore, alternative materials such as the titanate/ceria composite (La0.35Sr0.65TiO3–Ce0.5La0.5O2−δ) or (La0.75Sr0.25)0.95Mn0.5Cr0.5O3 (LSCM) have been proposed electrolysis cathodes. Both LSF and LSM/YSZ are reported as good anode candidates for electrolysis mode.<ref>{{cite journal|last=Laguna-Bercero|first=M.A. |author2=J.A. Kilner |author3=S.J. Skinner|title=Development of oxygen electrodes for reversible solid oxide fuel cells with scandia stabilized zirconia electrolytes|journal=Solid State Ionics|doi=10.1016/j.ssi.2010.01.003|year=2011|volume=192|pages=501–504}}</ref> 
Furthermore, higher operation temperature and higher absolute humidity ratio can result in lower area specific resistance.<ref>{{cite journal|last=Hauch|first=A.|author2=S. H. Jensen |author3=S. Ramousse |author4=M. Mogensen |title=Performance and Durability of Solid Oxide Electrolysis Cells|journal=Journal of the Electrochemical Society|date=18 July 2006|doi=10.1149/1.2216562|url=https://iopscience.iop.org/article/10.1149/1.2216562|volume=153|issue=9|pages=A1741|bibcode=2006JElS..153A1741H |s2cid=98331744 }}</ref>